FIG. 14 shows a conventional vertically articulated arm mechanism. The vertically articulated arm mechanism requires three-degree-of-freedom (x, y, z) regarding a position and three-degree-of-freedom (φ, θ, ψ) regarding a posture, which are generally realized by rotational joints J1, J2 and J3 called root three axes and rotational joints J4, J5 and J6 called wrist three axes. For example, a revolute joint is applied to joints J1, J4 and J6, and a bending joint is applied to joints J2, J3 and J5.
This type of the articulated arm mechanism has countless postures called singular points in which a degree of freedom in a specific direction is lost despite being within a movable range. The singular point posture causes uncontrollable condition due to countless solutions (sets of joint angles satisfying a certain coordinate position) that exist for a coordinate transformation operation of Cartesian coordinates and sets of joint angles, and causes “out-of-control” because an infinite angular velocity is required for the joint and a movement of the joint becomes rapidly fast in the vicinity of the singular point. Thus, in current situation, the problem is handled by avoiding the singular point or sacrificing a hand movement speed. For example, as the singular point, there is a so-called wrist singular point at which axes of rotation RA1, RA4 and RA6 of the revolute joints J1, J4 and J6 are made coaxial by setting all the bending joints J2, J3 and J5 to zero degree as shown in FIG. 15A; and in this situation, it is impossible to uniquely determine rotation angles of these revolute joints J1, J4 and J6. This situation similarly occurs when at least two revolute joints are aligned in the same axis. As other singular point postures, known are a so-called shoulder singular point at which an intersection point of the axes of rotation RA4, RA5 and RA6 of the joints J4, J5 and J6 of the wrist three axes is located on the axis of rotation RA1 of the revolute joint J1 forming an arm base as shown in FIG. 15B, and a so-called elbow singular point at which an intersection point of the axes of rotation RA4, RA5 and RA6 of the joints J4, J5 and J6 of the wrist three axes is located on a plane including axes of rotation RA2 and RA3 of bending joints J2 and J3 of the root three axes as shown in FIG. 15C.